1. Technical Field
This invention relates generally to a method and system for testing oil well cement samples, and, more specifically, to non-mechanically estimating the static gel strength of a cement slurry by measuring changes in the amplitude of an acoustic signal transmitted through the slurry under typical wellbore temperature and pressure conditions and correlating the measured signal attenuation data to actual static gel strength measurements.
2. Background
Most cement slurry formulations used in cementing oil, gas and geothermal wells are first tested for specific cement slurry properties conducted in a cement testing laboratory. To be meaningful, tests must simulate actual job conditions. Many aspects of oil well cementing are important, but the cement formulation is one of the few items which can be changed to modify the slurry performance under existing well conditions. The cement slurry is one of the most tested components pumped into a well.
The basic design of a cement slurry starts with determining what general properties are needed for predicted well conditions. The fracture or parting pressure of the weakest formation and cemerit column height determines maximum density of the cement slurry. The zone with the highest gas pressure gradient determines minimum density. The difference in the hydrostatic pressure and the highest formation pressure is the critical parameter that affects the cement slurry's ability to control the formation fluid flow into the cement column and is referred to as "overbalanced pressure". A low overbalanced pressure will require the cement slurry to have increased compressibility, early plastic state expansion, fluid loss control, modified gel strength profile, or possibly a complete change in job procedure. Other downhole parameters like salt zones and sensitive shales call for special additives to the slurry. Except for highly unusual situations, slurry specifications are met simply by additive selection based on previous experience or laboratory test results.
With the basic slurry formulation established, two physical properties of the slurry are of initial concern. The cement must remain fluid long enough to be pumped to its desired location downhole. Also, once the cement is in place it must set and develop an adequate compressive strength value within a specific time period. Thickening time tests and compressive strength tests account for most of the analysis conducted on cement slurries in field laboratories.
Thickening time measurements are generally determined using a high temperature-high pressure (HTHP) consistometer. This apparatus consists essentially of a rotating cylindrical slurry container equipped with a stationary paddle assembly, all enclosed in a pressure chamber capable of withstanding a pressure of 207 MPa (30,000 psi) and maximum temperatures to 204.degree. C. (400.degree. F.). A heating element is used to affect the temperature of the slurry at a maximum rate of 2.8.degree. C. (5.degree. F.)/minute. The thickness or consistency of the cement slurry is monitored by measuring the torque on the stationary stirring paddle and correlating the measured torque to amount of fluid time available for pumping the cement slurry.
Compressive strength measurements are best taken using what is known as the Ultrasonic Cement Analyzer (UCA). The UCA was developed to measure the compressive strength of a cement slurry as it sets when subjected to simulated oil field temperatures and pressures. It consists of a high temperature-high pressure autoclave, a heat jacket capable of heating rates up to 5.6.degree. C. (10.degree. F.) per minute, a pair of 400 kHz ultrasonic transducers for measuring the transit time of an acoustic signal transmitted through the slurry, plus associated hydraulic plumbing. The two transducers make transit time measurements through the cement as it sets. A short pulse on a lower transducer propagates through the cement to an upper transducer. Set time and compressive strength are calculated from measured transit time via empirically developed equations. U.S. Pat. Nos. 4,259,868 and 4,567,765 disclose the UCA in detail and are incorporated herein by reference.
Unfortunately thickening time tests and compressive strength tests do not tell the whole story. Thickening time is a test which only simulates actual job conditions up to the predicted placement time. After allowing for test accuracy variation, a thickening time longer than the placement time allows for some margin of safety but only for continuous pumping at a lower than predicted rate. Thickening time "safety factors" do not directly relate to how long a slurry can remain static and still be moved after an inadvertent or intentional shutdown during placement. With respect to what actually takes place downhole, a thickening time measurement provides information on what happens up to the end of placement time. A thickening time of six hours tells nothing about what change will occur when the slurry is allowed to remain static after pumping. The compressive strength test shows the degree of hydration and set that will occur 8, 12 or 24 hours after placement. Until recently, the in-between period was left to speculation.
An important phenomenon occurring in the cement slurry after it has been pumped downhole is static gel strength development. Static gel strength development is a by-product of the initial hydration process of the cement. The beginning of gel strength development signals the point at which the cement slurry starts its change from a true hydraulic fluid which transmits full hydrostatic pressure to a solid set material that has measurable compressive strength. This period of change is called the transition phase. During the transition phase the cement slurry is continually gaining gel strength. This gel strength is important for two reasons. First the static gel strength development determines the shut down safety factor on the job. If the cement slurry is stopped prior to placement then the gel strength allows the calculation of the pressure required to restart circulation. Secondly the static gel strength affects the pressure reduction and the fluid flow of gas or water into the cement filled annulus. This fluid flow problem will be the main focus of the static gel strength discussion due to its importance.
Static gel strength in the cement enables a potential pressure restriction to occur in a cement filled annulus. If volume decreases occur when gel strengths are present, the actual pore pressure in the cement can decrease. This pressure decrease can be severe enough to allow gas to enter the annulus. The total time of this transition period is critical when gas flow potential exists. If the transition time becomes longer, it allows more volume decreases to occur, and thus, more gas flow into the cement column. If the gel strength in the cement slurry is high enough and the fluid volume low enough then migration of gas is prevented.
In addition to gas flows through a cement slurry many in the industry are using static gel properties to control the flow of water. Some believe water flows through cement slurries to be the most critical problem encountered while drilling, for example, in deep water in the Gulf of Mexico. Static gel strength development can be quantified and utilized to design slurries that prevent undesirable water flow.
The next important phenomena that occurs after pumping and the onset of static gel strength development is set time. This is the point where compressive strength first begins to develop in the cement slurry. This signals the end of static gel strength development and the start of compressive strength development and will determine what is called the waiting-on-cement (WOC) time.
The present invention is primarily concerned with new methods and apparatus for determining the static gel strength characteristics of cement slurries.
Static gel strength (SGS) is basically a shear bond strength measurement derived from the pressure required to move a gelled fluid through a pipe or annulus at a micro rate. The unit compatible equation, EQU SGS=P(D/4L)
(or) EQU P=SGS.times.4(L/D)
is in fact a definition of static gel strength, as well as an application equation. The units of SGS in USA engineering units are lbs./100 ft.sup.2 when derived from the equation SGS=300 P.times.D/L where P (pressure)=psi, D (effective diameter, hole size minus the pipe size)=in. and L (length)=ft.
Different measurement systems have heretofore been employed for determining static gel strength. In one system a device similar to a consistometer is specifically designed for measuring static gel strength after a stirring period to simulate slurry placement. The equipment is designed to simulate downhole conditions and a low friction magnetic drive allows the slurry to be stirred while monitoring consistency during the stirring time. After simulating placement time, the motor is shutoff and a cord pulling system is attached to the magnetic drive head. Static gel strength is determined by continuously measuring the torque required to rotate a paddle at a very slow speed (0.5 to 2.0 degrees per minute). At such speeds, a magnetic drive has very low friction and accurate torque measurements can be made. In this system the torque measuring equipment consists of a cord pulling capstan or drum arrangement driven by a variable speed gear motor with the cord running through the pulley arrangement to a load cell and then to the magnetic drive of the stirring autoclave. This provides a method of accurate continuous rotation and a means for continuously recording the torque. The gel strength is then calculated from the torque measurement and the paddle geometry. The slow movement of the paddle allows static gel strength to be measured but does not inhibit gel strength development. With this device gel strength properties can be measured from a minimum of 4.8 Pa (10 lbs./100 ft.sup.2) up to a maximum of 480 Pa (1000 lbs./100 ft.sup.2). The well known Halliburton MACS analyzer is an example of this type of device.
Static gel strength has also been measured by determining the pressure drop across a length of tubing. The basic set-up of such an apparatus allows for the circulation of the test slurry through a small diameter tubing. After placement, the slurry is pressurized with water to the test pressure. A sensitive pressure drop transducer measures the pressure drop of the cement as it gels from the entrance to the exit of the tubing. As the cement gels, a corresponding pressure drop will be observed. The pressure drop and the static gel strength can be related using the above equations.
Static gel strength has also been determined with a shearometer device. This device is a thin wall cylinder that is placed in a sample of the cement slurry and allowed to remain static for a period of time. Weight is then applied to the top of the tube until the tube moves through the cement slurry. This method is conducted at atmospheric pressure but at temperature if the temperature is less than 200.degree. F.
One problem with prior art systems for determining SGS is that mechanical methods have a limited low-range torque measurement resolution and poor sensitivity. Mechanical methods and systems are also difficult to calibrate. Furthermore, tests such as the rotating paddle test may not provide a continuous measurement of SGS. In order to measure the static gel the gel is broken and only one data point is obtained.
While more accurate than a mechanical approach, pressure drop measurements of SGS as described above are not practical in most cases as it is difficult to analyze a large number of samples. Once a measurement is completed the tube is discarded and another is used for the next measurement. This is overly time consuming and expensive.
Tests such as the shearometer conducted at atmospheric pressure do not translate well to the characteristics of the cement slurries at downhole conditions (downhole pressure).
It is thus an object of the present invention to provide a non-mechanical method and apparatus for accurately determining the SGS of a cement slurry that increases the resolution and sensitivity of the measurement and provides a continuous measurement of SGS under downhole conditions.
It is another object of the invention that multiple samples of cement slurries be quickly and easily analyzed without undue expense.